Correct getProcAddress() handling in robustness tests

Correct getProcAddress() handling in robustness tests

The below robustness tests don't use the OpenGL ES extension name when
calling getProcAddress() in an OpenGL ES environment. This change fixes
it by removing the getProcAddress() calls and use gl.XXX() to call these
functions. gl.XXX() takes care of the context it is running in.

Affects:
KHR-NoContext.*.robustness.no_reset_notification
KHR-NoContext.*.robustness.getnuniform
KHR-NoContext.*.robustness.readnpixels

Components: OpenGL
VK-GL-CTS issue: 1267

Change-Id: I4ed18990626f1dd744eebf6b99c18a60a29dad5f